no COMPARATIVE ANATOMY AND PHYSIOLOGY. 



and examine a free-swimming Medusa (Fig. 10) we 

 find that the mouth and the stomach form a free 

 projection hanging downwards, sometimes in the shape 

 of a tube of some length \ around this mouth we again 

 find tentacles, and if we examine the first portion of 

 the gastric apparatus we find it is widened out to form 

 what may be called a stomach ; connected with this 

 last there are a large number of canals which channel 

 the substance of the disc of the umbrella, and carry 

 into it the nutriment prepared by the gastric cells ; 

 these canals have, therefore, a circulatory function, and 

 are, consequently, appropriately spoken of as the 

 gastrovascular canals. They either run simply 

 or are ramified, and are again brought into connection 

 with one another by opening into a canal which runs 

 round the edge of the umbrella ; from this canal 

 cavities sometimes pass into the tentacles which fringe 

 the margin of the disc. 



The tentacular processes set around the mouth are 

 often of considerable size, and are in certain forms 

 broken up into a number of processes ; in one group 

 (that of the Rhizostomidse) this is carried to an ex- 

 treme, for the oral tentacles take the place of the 

 mouth, which, in the adult, is closed up, and they 

 become provided with digestive cells and openings to 

 the exterior ; so that in these forms a number of small 

 secondary orifices take the place of the single large 

 primitive mouth. 



The other great division of the Ccelenterata, that 

 of the Anthozoa, presents us at once with an impor- 

 tant distinctive character ; for the mouth is not placed 

 on a projecting cone, but is depressed below the level of 

 the surrounding platform developed from the body wall 

 (Fig. 54). The second distinction is perhaps the more 

 important ; the tube into which the mouth leads is 

 widely open at the lower end ; in other words, we 

 have here the appearance not of a system of canals 



